1. Field of the Invention
The invention relates generally to RF filters, and more specifically to RF filters with one or more adjustable characteristics.
2. Description of Related Art
State-of-the-art RF filters are commonly implemented through the use of a plurality of parallel-resonant LC sections that are capacitively coupled to the source, to the load, and to each other. In many applications, it would be desirable to provide a mechanism by which the center frequency and/or the bandwidth of such a filter may be adjusted. To this end, various techniques have been used to provide filters having tunable characteristics. In general, these techniques involve changing the reactance of one or more filter components. For example, many low-cost portable AM/FM radios use a mechanical multi-gang variable capacitor, in combination with fixed inductors, to provide tuning across the AM and FM broadcast bands.
Electronically-tunable filter designs incorporate varactor diodes into the parallel-resonant LC sections, and/or use varactor diodes to control the capacitive coupling elements of the filter. Varactor diodes may be conceptualized as voltage-controlled variable capacitors, because the capacitance provided by a varactor diode is roughly proportional to the reverse DC bias applied to the varactor diode. Varactor diodes have been used to make RF filters having tunable center frequencies and/or tunable bandwidth. In a typical filter arrangement providing for adjustment of center frequency, each parallel-resonant LC section of the filter includes a tuning element comprising one or more varactor diodes. The capacitance of the diodes is set to a desired value by adjusting a DC control voltage, thereby "tuning" the RF filter to a desired center frequency. In a typical filter arrangement providing for adjustment of bandwidth, one or more of the capacitive coupling elements of the filter includes a varactor diode for adjusting the amount of coupling between adjacent parallel-resonant sections of the filter, and/or for adjusting the coupling between the filter and a source/load element.
In many system applications, it would be desirable to minimize the number of varactor diodes that are used in an electronically-tunable filter while, at the same time, providing a filter having adjustable bandwidth and adjustable center frequency. In order to provide an electronically-tunable filter with these adjustable properties, prior art designs require the use of a first set of varactor diodes to adjust the bandwidth, and another set of varactor diodes to adjust the center frequency. Since varactor diodes add cost to a filter design, it is desirable to keep the number of varactor diodes in a filter design to a minimum.
As the number of varactor diodes in a filter is increased, it becomes increasingly difficult to properly align the filter. Since two varactor diodes will generally not exhibit identical voltage-versus-capacitance characteristics, tracking mechanisms are used to compensate for inherent device-to-device variations, such that, for example, all resonant elements will be tuned to the same frequency at a given varactor DC supply voltage. These tracking mechanisms typically take the form of variable trimmer capacitors and/or variable resistors (potentiometers). As the number of varactor diodes in a filter is increased, it becomes increasingly difficult and time-consuming to properly adjust filter tracking. For these reasons, it would be desirable to minimize the number of varactor diodes used in an adjustable filter design.
The capacitance provided by a varactor diode is linearly related to the applied reverse bias voltage only over a certain range of reverse bias voltages. Outside of this voltage range, the varactor diode exhibits nonlinear properties and provides a capacitance with an insufficiently high Q (i.e., the capacitance is swamped by too much series resistance). Even within the linear operating region of the varactor diode, the Q of a varactor diode is often lower than that of a conventional air-dielectric or mica-dielectric variable capacitor. If a filter design requires relatively high-Q elements, it may not be possible to achieve a desired level of performance with varactor diodes. In a similar vein, varactor diodes are more vulnerable to relatively high levels of applied RF energy than is the case with mechanical capacitors. When confronted with strong RF input signals, varactor diodes can introduce intermodulation and other spurious products into a filtered signal.
One technique for providing a tunable resonator element, while at the same time overcoming the disadvantages of varactor diodes, is described in U.S. Pat. No. 5,065,121 issued to Masood Ghadaksaz on Nov. 12, 1991 (hereinafter, Ghadaksaz). Ghadaksaz discloses a single resonator element that has a selectable center frequency. The center frequency is changed by electronically switching a plurality of fixed inductive and capacitive elements into a resonant circuit. However, Ghadasaz does not describe any mechanism for tuning the bandwidth of the resonator element, nor does Ghadasaz describe how a plurality of resonator elements could be combined with other reactive elements to form an RF filter. Therefore, there is a need for a tunable filter design which minimizes the number of tunable filter elements while providing a mechanism for adjusting center frequency and bandwidth.